Effects of Selected Plant-Derived By-Products as Partial Soybean Meal Replacements on Nutrient Utilization, Cecal Microbiota, and Performance of Aged Laying Hens

Soybean meal (SBM) is a major protein source in commercial layer diets; however, fluctuations in SBM prices and supply have increased the need for alternative protein sources in poultry production. Plant processing by-products are considered promising feed resources because of their abundant availability and nutritional value. However, the effects of partially replacing SBM with different plant-derived by-products on digestive function, gut microbiota, nutrient utilization, productive performance, and egg quality in aged laying hens remain insufficiently characterized. Therefore, this study aimed to evaluate the effects of partially replacing SBM with different plant-derived by-products on gastrointestinal digestive enzyme activities, cecal microbial community composition, apparent nutrient utilization, productive performance, and egg quality in aged laying hens. A total of 270 Hy-Line Brown laying hens at 80 weeks of age were randomly assigned to five dietary treatments, with six replicates per treatment and nine hens per replicate. Digestive enzyme activities and cecal microbiota were assessed using three birds per treatment. The control group was fed a corn-SBM basal diet, whereas the experimental groups were fed diets in which SBM was partially replaced with 10% palm kernel meak (PKM, T1), 12% rice bran meal (RBM, T2), 15% sunflower meal (SFM, T3), or 8% Zanthoxylum bungeanum seeds (ZBS, T4), respectively. The results showed that digestive enzyme activities varied among dietary treatments, with responses depending on the type of enzyme and gastrointestinal segment examined. Cecal microbiota analysis revealed no significant differences in alpha diversity among dietary treatments (p > 0.05), whereas overall microbial community composition differed significantly (PERMANOVA, p = 0.015). Differences in the relative abundances of individual bacterial taxa were also observed, but none remained significant after multiple-testing correction. Furthermore, the T3 group exhibited significantly higher apparent utilization of all six measured nutrients than the control group, whereas the other treatments showed mixed, nutrient-specific responses. Dietary treatments affected laying rate and eggshell thickness (p < 0.05), whereas average egg weight, average daily feed intake, feed-to-egg ratio, and other measured egg quality parameters were not significantly affected (p > 0.05). Overall, the four plant-derived by-products produced variable responses across the measured outcomes, suggesting ingredient-specific potential as partial substitutes for SBM under the dietary formulations and inclusion levels evaluated. However, their equivalent nutritional suitability cannot be established from the present findings, and further studies with larger sample sizes and multiple inclusion levels are warranted.